The present invention relates to a new total hip prosthesis with primary fixation.
As conventionally known, there are essentially two categories of hip prosthesis.
In the first category, the prosthesis is cemented. But this solution presents two types of problems:
problems during surgery due to anaesthetic shocks caused by the cement itself; and
problems after surgery, namely risks of infection where the bone joins up with the cement, loosenings and difficulties in re-operating, if the prosthesis needs to be changed eventually, because the bone, which has been drilled in, has become brittle.
The second category, which is now progressively spreading, is called "prosthesis with primary fixation". In this type of prosthesis, the pin is force-fitted into the bone by adjustment and the bone re-formation occurs according to a special design of that pin. This type of prosthesis is essentially constituted by three principal parts, namely, in the right order:
a femoral pin, for insertion into the femur;
a cotyloidal cupule, for engagement into the acetabulum of the treated hip;
a prosthetic neck, covered with a sphere, which joins the pin to the cupule and is intended to make the articulation proper.
Numerous solutions have heretofore been proposed to produce such prostheses.
For example, it has been suggested to screw the femoral pin into the femur and, like a screw, to give to said pin a slightly truncated shape so as to make the screwing operation easier (as described for example, in European Pat. No. 0010527 and in French Pat. No. 2 295 729). Such conicity however, is often ill-adapted to the shape of the femur, this causing an inaccurate adjustment, hence a wrong distribution of the stresses. Moreover, the threads used up to now, have been found to create risks of unscrewing, either due to the insufficient length of the thread or due to the drawing-back effect on the supporting flange. This causes postoperative / pains possibly due to stress peaks occuring at the level of this thread.
European Pat. No. 000549 and French Pat. No. 2 481 596 both propose to provide on the head of the pin which is not engaged into the femur, a co-axial flange, of which the lower face rests against the femur, whereas the upper face is provided with position locating and holding means, this permitting the location and holding of the angular position of the prosthetic neck with respect to the pin during assembly. Although this solution offers a great number of advantages, in practice, as the flange rests on the reduction, this causes an important resection of the bone and can give rise to fractures on the upper end of the femur, and particularly in the zone known as the "trochanter". Moreover, from the very structure of this flange, it is impossible to obtain an ideal orientation of every case of anteversion of the femoral prosthetic neck.
It is the object of the present invention to overcome the above disadvantages by proposing a total hip prosthesis with primary fixation which eliminates all of the aforesaid problems, namely a total hip prosthesis with primary fixation which:
is better adapted to the shape of the femur, hence which can be better adjusted;
reduces the stress peaks on the level of the thread;
improves the primary fixation and reduces resection of the upper end of the femur;
and finally, which permits an ideal orientation of the anteversion of the prosthetic neck.
The total hip prosthesis with primary fixation according to the invention, is of the type formed of three principal parts:
a threaded femoral pin, designed to be screwed into the femur,
a cotyloidal cupule, designed to be engaged in the cotyloidal cavity of the treated hip; and
a prosthetic neck, topped by a sphere joining the head of the pin to the cupule and designed to ensure the articulation proper, said prosthesis being characterized in that the threaded femoral pin has a tulip-shaped tapered longitudinal section, widening from the bottom end towards the head with progressively variable radius.
In other words, the invention relates to a total hip prosthesis with primary fixation, wherein the threaded femoral pin has a special tulip-shaped anatomical profile, with progressively variable radius.
By "profile with progressively variable radius" is meant a longitudinal profile of which the radius of curvature varies permanently and progressively from the point of the pin, where it reaches its maximum, to its head, hence a profile of which the radius of the section increases progressively from the point to the head.
Advantageously, in practice:
the section of the pin is circular;
the thread of the threaded femoral pin, as well as that of the cupule, have a bevelled trapezoidal section with advantageously rounded angles.
In said trapezoidal thread, the upper face is less inclined with respect to the direction of the thread than the lower face with respect to the same plane of the pin cross-section;
the angle of inclination of the upper face with respect to the plane of said cross-section is between 15.degree. and 25.degree., and preferably around 20.degree., and the angle of inclination of the upper face with respect to that same plane, is between 10.degree. and 20.degree., and preferably around 15.degree.;
the threaded pin is also provided with longitudinal slots situated according to generatrices, the angles of incidence of which are inclined particularly in both directions, in order to help the screwing and unscrewing operation;
the fitting of the prosthetic neck over the threaded head and over the sphere-shaped prosthetic articulation head is achieved by means of two conical bores of low-inclination, respectively male and female bores.